Optical fiber connector using photocurable adhesive

ABSTRACT

A fiber optic connector including a holder for one ore more optical cables, each optical cable having an optical fiber surrounded by strengthening fibers, both of which are surrounded by at least one polymeric coating layer. The cable is stripped in a layerwise manner such that an area of exposed fiber is succeeded by an area of exposed strengthening fibers, succeeded by an area of coated cable. A visible light curable adhesive is injected into the holder and cured by exposure to visible light for up to about 30 seconds, bonding the optical fiber layers and the holder into a unified structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an optical fiber connector and the use ofradiation in the visible portion of the spectrum to cure a photocurableadhesive in an optical fiber connector to bond the optical fiber, thereinforcing fibers and the optical holder into a unified structure, inorder to provide an improved connection between two optical fibers or anoptical fiber and an optoelectronic component.

2. Description of the Art

Optical fibers have replaced copper wire as the preferred medium forcarrying telecommunications signals. As with copper wire, it isnecessary to provide for the interconnection of optical fibers duringinstallation, repair or replacement of the fibers, and to terminate thefibers onto active optical devices. There are generally two kinds ofinterconnection devices, splices and connectors. The term "splice"usually refers to a device, which provides a permanent connectionbetween a pair of optical fibers. The term "connector," in contrast,usually refers to a device, which may be engaged and disengagedrepeatedly, often with a different plug or receptacle. A connector mayalso refer to the plug portion of a fiber termination, which is attachedto an optical device. Optical devices include, for example, opticalsensors (photoelectric diodes) and light sources (LED's, laser diodes).The termination of an optical fiber may be indirect, i.e., the fiber maybe connected to some other (passive) optical device such as a beamsplitter or polarizer, before the light beam is directed to the activeoptical device. The present invention is generally directed to aconnector, although this term should not be construed in a limitingsense since the present invention may inherently provide a permanent, aswell as temporary connection or termination.

In the fiber optic connector described in U.S. Pat. No. 5,381,498, theconnector has a plug and a receptacle, the plug having afiber-receiving, V-shaped groove for each fiber to be interconnected,with the end of the fiber terminating in the middle of the groove. Thereceptacle has a plate which retracts as the plug is inserted, wherebyanother fiber is lowered into the V-groove of the plug. Upon fullinsertion of the plug, the two fiber ends are in contact, and the fibersecured to the receptacle is elastically deformed to maintain acontinuous compressive load between the terminal ends of the fibers. Theconnector provides for the quick disconnection and reconnection of aplurality of optical fiber pairs, without the use of ferrules or otheralignment members. High strength fiber may be used to withstand repeatedinsertions and bowing of the fibers. The exact lengths of fibers (i.e.,the relative locations of their terminal ends in the plug andreceptacle) are not critical since tolerance is provided by the slacktaken up in the bowed receptacle fiber (the terminal portion of thefiber secured to the plug does not bow, but always remains straight).The ends of the fibers may be prepared by simply cleaving and beveling;the end faces may optionally be cleaved at an angle (i.e.,non-orthogonal to the fiber axis) to reduce signal reflections.

Many fiber optic splices employ plate elements having fiber-receivinggrooves, with mechanisms for clamping the terminal ends of the fibers ina common groove. Some of these devices are designed to interconnect aplurality of pairs of fibers, such as the splice shown in U.S. Pat. No.5,151,964. In U.S. Pat. No. 4,028,162, fibers approach alignment groovesat a glancing angle and are held temporarily while a connector plate isadhered to the interconnected fibers. For other examples of techniquesinvolving bowed fibers entering alignment grooves, see U.S. Pat. Nos.4,077,702, 4,148,559, 4,322,127 and 5,080,461, and French PatentApplication No. 2,660,442. Some of the connector designs using theprinciple of bowing a fiber into a fiber-alignment groove are rathercomplex and require many parts, such as the designs seen in U.S. Pat.Nos. 4,045,121, 4,218,113 and 4,767,180.

In order to provide a termination or interconnection with the requiredstrength without damaging the system, the fibers must be secured to theconnector body to inhibit or at least reduce the relative movementbetween the optical fiber and its outer sleeve.

Such an attachment system may be mechanical, such as a clamp or set ofclamps or it may be a type of adhesive. A mechanical system may alsoinclude strength members such as layers of stranded steel wire, asdisclosed in U.S. Pat. No. 5,539,849.

Useful adhesives for termination must be capable of bonding to the outersurface of the fiber, which may be formed from materials such as glass,epoxy silicones, and the like. It also must be capable of bonding toother materials used in fiber optic cables and their terminations, suchas polymeric coating layers, and strengthening fibers used to surroundthe optical fibers, and plastics from which the holder is formed. Thestrengthening fibers are typically aromatic polyamide fibers derivedfrom p-phenylenediamine and terephthaloyl chloride, availablecommercially as Nomex® or Kevlar®.

U.S. Pat. No. 4,699,462 discloses a method for forming a terminationbetween a fiber optic cable having a centrally positioned optical fiber,a plurality of surrounding reinforcements, and a component housing. Anadhesive, preferably a heat activated adhesive, is applied within thetermination and heat shrink tubing is applied in order to force thereinforcement fibers into adhesive engagement with the adhesive layer.Bond formation occurs primarily at the interface between the cladding onthe optical fiber core, and reinforcement strands. The adhesive does notprovide bonding to the heat shrink tubing; it is present to providereinforcement to the termination.

U.S. Pat. No. 5,058,984 discloses a fiber optic cable connectorcomprising a plastic outer sleeve to be optically couple to anotheroptical fiber cable, carrying at one end, connection means for coupling,a tubular gripping member which adheres the fiber to the outer sleeve orholder, which is deformed by application of force so as to grip the endportion of the plastic outer sleeve and a ferrule mounted within theother end of the connector body supporting an exposed end portion of anoptical fiber. The optical fiber is adhered to the ferrule with adhesivematerial. The ferrule is ceramic and the exposed end of the fiber is setwith a light curable resin, generally blue light where the ferrule isformed of zirconia. This allows a setting time to be reduced to about 60seconds. It is specifically disclosed that an irradiation curableadhesive might not adhere sufficiently strongly to the plastics outersleeve; therefore the adhesive material is used to secure the endportion within the ferrule, and does not need to provide any adhesive tothe plastic outer sleeve.

As can be seen, even with the use of adhesives, conventional fiber opticconnector assemblies have required the use of additional positioning orbonding means in order to resist disruptive force. It would be verydesirable to eliminate such means and be able to provide a unifiedsystem wherein the bond is formed solely from an adhesive which adheresthe optical fiber to the outer holder of the connector, and adheres tothe fiber as well as the coating and strengthening fibers withoutrequiring additional positioning means such as heat shrink tubing,gripping members and the like.

U.S. Pat. No. 5,525,648 discloses a treatment method for application todentin and cervical enamel or adhering to hard tissue in a high humidityenvironment. Primer compositions disclosed bond strongly to dentin, andexhibit high shear strength, and include an acid and a film former,which are applied and then hardened. A wide variety of acids are useful,including organic, inorganic, solid and liquid acids. Useful filmformers are water dispersible and may be selected from many polymers,monomers and mixtures. After standing time to achieve priming, theprimer, optionally, with an additional layer of film former is thenhardened by use of a polymerization catalyst.

U.S. Pat. No. 5,545,676 discloses a three component or ternaryphotoinitiator system for use in additional polymerization. A variety ofacrylate monomers are disclosed. The system is disclosed to provide acombination of cure speed, cure depth and shelf life, and is disclosedto be useful in color profiling systems, curable inks, printing plates,photoresists, coated abrasives, photocurable adhesives and composites,e.g., for dentistry or autobody repair.

The present invention provides a fiber optic connector using a visiblelight photocurable adhesive composition to bond the optical fiber(s),the strengthening fibers surrounding the fiber and a coating layerthereover into a unified structure. The system provides a fast cure,good depth of cure, and a safe, low energy means for providing theconnection, which can be easily accomplished by a tradesperson.

SUMMARY OF THE INVENTION

The invention provides an optical fiber connector comprising an opticalfiber, and a termination or interconnection bonded in place solely bymeans of a visible light photocurable adhesive.

Specifically, the invention provides a fiber optic connector including

a) at least one fiber optic cable comprising at least one optical fibersurrounded by a plurality of strengthening fibers, both of which aresurrounded by at least one polymeric coating layer, a portion of saidcable being stripped in a layerwise manner such that an area of exposedfiber is succeeded by an area of exposed strengthening fibers, succeededby an area of coated cable;

b) a holder for the optical cable, and

c) a visible light curable adhesive injected into the holder, theoptical fiber, the strengthening fibers and the holder being bonded intoa unified structure by means of exposure to visible light for a periodof up to about 30 seconds.

Preferably, the adhesive comprises at least one acrylate monomer. Theadhesive is curable by visible radiation having a wavelength betweenabout 400 and about 700 nanometers; preferably from about 400 to about600 nanometers, which includes a portion of the blue and the green areaof the spectrum, most preferably between about 500 and about 600nanometers. Curing time is less than about 30 seconds, preferably lessthan about 25 seconds. The adhesive bonds strongly to the fiber, coatingand holder to form a unified structure, such that the structure is noteasily disrupted when force is applied to the connector, or to thestrengthening fibers.

In an alternate embodiment, the adhesive is a photocurable adhesivewhich is curable by exposure to radiation having wavelengths of fromabout 700 nm to about 1100 nm, in the near-infrared region.

Preferred optical fiber connector assemblies of the invention comprisean adhesive having a ternary photoinitiator system comprising anelectron donor, a sensitizer and a diaryliodonium salt.

The invention also provides a method of bonding interconnections orterminations in optical fiber connectors and cables wherein thetermination or interconnection is bonded in place solely by means of avisible light photocurable adhesive.

As used herein, these terms have the following meanings.

1. The term "visible light" means electromagnetic radiation havingwavelengths between about 400 nm and about 700 nm.

2. The term "near-infrared", abbreviated "nIR", means electromagneticradiation having wavelengths between about 700 nm and about 1000 nm.

3. The term "plug" means an article, which is present in a connector,for retaining and selectively aligning the first optical fiber in aconnector. Plugs are insertable into the receptacle to form a connectionor termination.

4. The term "receptacle" means an article present in a connector, forretaining and selectively aligning the second optical fiber in aconnection

5. The term "holder" means that portion of the plug which holds thefirst optical fiber in place.

6. The term "connector" means an article for forcing the end of a firstoptical fiber towards an end of a second optical fiber in contact in theend of the first optical fiber. A connector includes a plug and areceptacle.

7. The term "unified structure" as used herein refers to the conditionwherein components, including the optical fiber, plurality ofreinforcing fibers and the holder are joined in a bonded, fixedrelationship held together by photocurable adhesive that has beenexposed to actinic radiation of suitable wavelength for adhesivepolymerization.

8. The terms "termination" and "connection" mean the point at which afirst optical fiber is forced into contact with either a second opticalfiber or an optoelectronic device.

9. The term "(meth)acrylate" includes both the acrylate and themethacrylate.

As used herein, all parts, percents, and ratios are by weight, unlessspecifically stated otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will best be understood by reference to the accompanyingdrawings, wherein:

FIG. 1 is a side view of a longitudinal section of one embodiment of thepresent invention, depicting a fiber optic connector including a plugand receptacle;

FIG. 2 is a perspective view of the plug and receptacle of FIG. 1, witha partial section revealing the bowed fibers in the plug interior.

DETAILED DESCRIPTION OF THE INVENTION

Optical fiber assemblies of the invention comprise at least one opticalfiber in a holder, adhered to the holder by means of a photocurableadhesive. While previous systems using adhesives have required otherpositioning devices such as heat shrinkable tubing, "gripping members"and the like, the assemblies of the invention develop a strong bondbetween the adhesive and the various portions of the connector such asthe holder, the fiber optical cable coating, the strengthening membersand the optical fiber, forming a unified structure.

Adhesive systems useful in assemblies of the invention are photocurablein the visible area of the spectrum, and can be applied as one or twopart adhesives. The adhesives may be provided already in the fiberassembly, or they may be provided separately, in syringe-likeapplicators, to enable application in the field.

A wide variety of monomers can be photopolymerized to form theconnection or termination in the connector of the invention.

Suitable monomers contain at least one ethylenically-unsaturated doublebond, and are capable of undergoing addition polymerization. Themolecular weight may vary, and the "monomers" discussed may includeoligomers.

One preferred monomer is formed by combining a dimethacrylate derivedfrom the reaction methacrylic acid and the diglycidyl ether of bisphenolA (BISGMA)with a hydrophilic monomer such as hydroxypropyl methacrylate,hydroxyethyl methacrylate (HEMA), or methacrylic acid. Such monomersinclude mono-, di-, or poly(meth)- acrylates such as methyl(meth)acrylate, ethyl (meth)acrylate, isopropyl (meth)acrylate, n-hexyl(meth)acrylate, stearyl (meth)acrylate, allyl (meth)acrylate, glyceroldi(meth)acrylate, glycerol tri(meth)acrylate ethylene glycoldi(meth)acrylate, diethyleneglycol di(meth)acrylate, triethyleneglycoldi(meth)acrylate, diethyleneglycol di(meth)acrylate, triethyleneglycoldimethacrylate, 1,3-propanediol di(meth)acrylate, trimethylolpropanetri(meth)acrylate, 1,2,4-butanetriol tri(meth)acrylate,1,4-cyclohexanediol diacrylate, pentaerythritol tetramethacrylate,sorbitol hexa(meth)acrylate,bis-1-(2-acryloxy)]-p-ethyoxyphenylclimethyl methane,bis[1-(3-acryloxy-2-hydroxy)]-p-propoxyphenyldimethylmethane,trihydroxyethyl-isocyanurate tri(meth)acrylate; the bis-acrylates andbis-methacrylates of polyethylene glycols having molecular weightsbetween about 200 and about 500; copolymerizable mixtures of acrylatemonomers such as those disclosed in U.S. Pat. No. 4,652,274, andacrylated oligomers such as those disclosed in U.S. Pat. No. 4,642,126,both of which are incorporated herein by reference; unsaturated amidessuch as methylene bis-acrylamide, methylene bis-methacrylamide,1,6-hexamethylene bis-acrylamide, diethylene triamine tris-acrylamideand beta-methyacrylaminoethyl methacrylate, and vinyl compounds such asstyrene, diallyl phthalate, divinyl succinate, divinyl adipate anddivinylphthalate. The adhesive may contain only one type of monomer ormixtures of two or more monomers may be used.

The photoinitiator system is one, which is capable of light absorptionin the visible range, i.e., between about 400 nm and about 700 nm. Inpreferred assemblies of the invention, the photoinitiator absorbs lightbetween about 400 nm and about 600 nm, more preferably between 500 nm toabout 600 nm, in the green and a portion of the blue portion of thespectrum. Components in the photoinitiator system include at least oneinitiator and at least one sensitizer.

Useful sensitizers should be soluble in the monomer and are capable oflight absorption in the appropriate wavelengths. The sensitizer is alsopreferably capable of sensitizing2-methyl-4,5-bis(trichloromethyl)-s-triazine, according to the testprocedure described in U.S. Pat. No. 3,729,313, incorporated herein byreference. Preferably, the sensitizer is also shelf stable forreasonable periods of time.

Suitable sensitizers are believed to include compounds in the followingcategories: ketones, coumarin dyes (e.g., keto-coumarins), xanthenedyes, acridine dyes, thiazole dyes, thiazine dyes, oxazine dyes, azinedyes, aminoketone dyes, porphyrins, aromatic polycyclic hydrocarbons,p-substituted aminostyryl ketone compounds, aminotriaryl methanes,merocyanines, squarylium dyes and pyridinium dyes. Ketones (e.g.,monoketones or alpha-diketones), ketocoumarins, aminoarylketones andp-substituted aminostyryl ketone compounds are preferred sensitizers.For applications requiring high sensitivity, it is preferred to employ asensitizer containing a julolidinyl moiety. For applications requiringdeep cure (e.g., where the coating or strengthening fibers attenuateradiation of similar wavelengths), it is preferred to employ sensitizershaving an extinction coefficient below about 1000, more preferably belowabout 100, at the desired wavelength of irradiation forphotopolymerization.

By way of example, a preferred class of ketone sensitizers has theformula:

    ACO(X).sub.b B

where X is CO or CR¹ R², where R¹ and R² can be the same or different,and can be hydrogen, alkyl, alkaryl or aralkyl, b is one or zero, and Aand B can be the same or different and can be substituted (having one ormore non-interfering substituents) or unsubstituted aryl, alkyl,alkaryl, or aralkyl groups, or together A and B can form a cyclicstructure which can be a substituted or unsubstituted cycloaliphatic,aromatic heteroaromatic or fused aromatic ring.

Suitable ketones of the above formula include monoketones (b=0) such as2,2-, 4,4- or 2,4-dihydroxybenzophenone, di-2-pyridyl ketone,di-2-furanyl ketone, di-2-thiophenyl ketone, benzoin, fluorenones,quinones, e.g., chloroquinone, 2-aza-3-carboxy-9-fluorenone, and thelike, chalcone, Michler's ketone, 2-fluoro-9-fluorenone,2-chlorothioxanthone, acetophenone, benzophenone, 1- or2-acetonaphthone, 9-acetylantracene, 2-, 3- or 9-acetylphenanthrene,4-acetylbiphenyl, propiophenone, n-butyrophenone, valerophenone, 2-, 3-or 4-acetylpyridine, 3-acetylcoumarin and the like. Suitable diketonesinclude aralkyldiketones such as anthraquinone, phenanthrenequinone, o-,m- and p-diacetylbenzene, 1,3-, 1,4-, 1,5-, 1,6-, 1,7- and1,8-diacetylnaphthalene, 1,5-, 1,8- and 9,10-diacetylanthracene, and thelike. Suitable α-diketones (b=1 and X=CO) include 2,3-butanedione,2,3-pentanedione, 2,3-hexanedione, 3,4-hexanedione, 2,3-heptanedione,3,4-heptanedione, 2,3-octanedione, 4,5-octanedione, benzil, 2,2'-, 3,3'-and 4,4'-dihydroxylbenzil, furil, di-3,3'-indolylethanedione,2,3-bornanedione (camphorquinone), 1,2-cyclohexanedione,1,2-naphthaquinone, acenaphthaquinone, and the like.

Other preferred sensitizers include Rose Bengal, Rose Bengale,Fluorescein, Eosin Yellow, Eosin Y, Ethyl Eosin, Eosin Bluish,Erythrosin Yellowish Blend, 4',5'-Dibromofluorescein.

The photoinitiator system also includes an electron donor. A widevariety of donors can be used; the donor should be soluble in themonomer, and have good shelf stability. Suitable donors are capable ofincreasing the speed of cure or depth of cure of a composition uponexposure to light of the desired wavelength. The donor has an oxidationpotential greater than zero, and less than or equal to the oxidationpotential of p-dimethoxybenzene. Preferable the oxidation potential isbetween about 0.5 and 1 volts vs. A saturated calomel electrode(S.C.E.). Values may be measured experimentally or obtained fromreferences such as N. L. Weinburg, Ed., Technique of ElectroorganicSynthesis Part II Techniques of Chemistry, Vol. V (1975) and the like.

Preferred donors include amines (including aminoaldehydes andaminosilanes), amides (including phosphoramides), ethers (includingthioether), ureas (including thioureas), ferrocene, sulfinic acids andtheir salts, salts of ferrocyanide, ascorbic acid and its salts,dithiocarbamic acid and its salts, salts of xanthates, salts of ethylenediamine tetraacetic acid, and salts of tetraphenylboronic acid. Thedonor can be unsubstituted or substituted with one or morenon-interfering substituents. Particularly preferred donors contain anelectron donor atom such as a nitrogen, oxygen, phosphorus, or sulfuratom, and an abstractable hydrogen atom bonded to a carbon or siliconatom alpha to the electron donor atom.

Preferred amine donor compounds include alkyl-, aryl-, alkaryl- andaralkyl- amines such as methylamine, ethylamine, propylamine,butylamine, triethanolamine, amylamine, hexylamine, 2,4-dimethylaniline,2,3-dimethylaniline, o-, m- and p-toluidine, benzylamine, aminopyridine,N,N'-dimethylethylenediamine, N,N'-diethylethylenediamine,N,N'-dibenzylethylenediamine, N,N'-diethyl-1,3-propanediamine,N,N'-diethyl-2-butene- 1,4-diamine, N,N'-dimethyl- 1,6-hexanediamine,piperazine, 4,4'-trimethylenedipiperidine, 4,4'-ethylenedipiperidine,p-N,N-dimethyl-aminophenethanol and p-N-dimethylaminobenzonitrile;aminoaldehydes such as p-N,N-dimethylaminobenzaldehyde,p-N,N-diethylaminobenzaldehyde, 9-julolidine carboxaldehyde and4-morpholinobenzaldehyde; and aminosilanes such astrimethylsilylmorpholine, trimethylsilylpiperidine,bis(dimethylamino)diphenylsilane, tris(dimethylamino)methylsilane,N,N-diethylaminotrimethylsilane, tris(dimethylamino)phenylsilane,tris(methylsilyl)amine, tris(dimethylsilyl)amine,bis(dimethylsilyl)amine, N,N-bis(dimethylsilyl)aniline,N-phenyl-N-dimethylsilylaniline and N,N-dimethyl-N-dimethysilylamine.Tertiary aromatic alkylamines, particularly those having at least oneelectron-withdrawing group on the aromatic ring, have been found toprovide especially good shelf stability. Good shelf stability has alsobeen obtained using amines that are solids at room temperature.

Preferred amide donor compounds include N,N-dimethylacetamide,N,N-diethylacetamide, N-methyl-N-phenylacetamide,hexamethylphosphoramide, hexaethylphosphoramide,hexapropylphosphoramide, trimorpholinophosphine oxide andtripiperidinophosphine oxide.

Suitable ether donor compounds include 4,4'-dimethoxybiphenyl,1,2,4-trimethoxybenzene and 1,2,4,5-tetramethoxybenzene.

Suitable urea donor compounds include N,N'-dimethylurea,N,N-dimethylurea, N',N'-diphenylurea, tetramethylthiourea,tetraethylthiourea, tetra-n-butylthiourea, N,N-di-n-butylthiourea,N,N'-di-n-butylthiourea, N,N-diphenylthiourea andN,N'-diphenyl-N,N'-diethylthiourea.

In one embodiment of the invention, the photoinitiator system is aternary system, according to U.S. Pat. No. 5,545,676, incorporatedherein by reference. In such a three component system, the additionalcomponent is a diaryliodonium salt. The iodonium salt should also besoluble in the monomer and be shelf stable when dissolved therein in thepresence of the sensitizer and donor. Accordingly an election of aparticular iodonium salt may depend to some extent on the monomersselected, and the other portions of the photoinitiator system. Suchternary system must contain these three parts; however, it may containmore than one sensitizer or electron donor, if desired.

Useful salts are those disclosed in U.S. Pat. Nos. 3,729,313, 3,741,769,3,808,006, 4,250,053, and 4,394,403, the iodonium salt portions of thedisclosures of which are incorporated herein by reference.

Preferred iodonium salts include diphenyliodonium chloride,diphenyliodonium hexafluorophosphate and diphenyliodoniumtetrafluorborate.

The adhesive of the present invention may be used with a variety ofdifferent connector designs. FIGS. 1 and 2 illustrate one embodiment 10of a fiber optic connector in accordance with the present invention. Theconnector 10 is comprised of an elongate plug 12 and a socket orreceptacle 14. FIG. 1 is a longitudinal section of connector 10 showingplug 12 fully inserted in receptacle 14, and receptacle 14 mounted on asupport surface or bulkhead 16. FIG. 2 is a perspective view withbulkhead 16 omitted, also with a partial longitudinal section toillustrate the interior of the connector. The depicted embodimentprovides for the interconnection of two pairs of fibers, but thoseskilled in the art will appreciate that the inventive concepts describedherein extend to single pair interconnection as well as interconnectionof a multiplicity of pairs.

The plug 12 includes a fiber holder 18 which may be constructed of twoclamping elements or blocks 20 and 22, and a plug body or shroud 24which is attached to fiber holder 18. Optical fibers 30 and 32 which areto be interconnected or terminated pass through holder 18 and into thehollow interior of shroud 24. The terminal portions of the fibers arebare, that is, they are not affixed to any alignment member such as aferrule. The shroud 24 therefore serves not only to assist in physicallylocating plug 12 in receptacle 14, but also to provide protection forthe otherwise exposed terminal portions of the fibers (the shroud couldbe made retractable to fully expose the fiber tips, if required). Theholder 18 has fiber-receiving grooves 34 formed in the adjacent surfacesof blocks 20 and 22; these two components may be identical parts.

The optical fibers are secured to holder 18 by use of the adhesiveheretofore described. The adhesive is injected through holes in theconnector for that purpose, and cured. The adhesive should adhere to thecable coating, which is typically epoxy silicone, polyolefin orpolyvinylchloride, where such is still present as well as thestrengthening fibers, e.g., Kevlar®, in those areas where the outercoating has been stripped away, and finally to the fiber itself to holdthe fiber in position.

The holder 18 may have an extension 36 surrounding the fibers foradditional strain relief and clamping. A boot 38 may be provided forfurther strain relief and capturing of the strengthening fibers in thefiber cable (KEVLAR® strands), and to assist in handling plug 12. Thestrengthening fibers need not be crimped, but they may be adhered intothe unified structure of the holder by means of the same adhesive asused to secure the fiber to the holder, as described supra. Strainrelief of the strengthening members is attained by a force fit ofstraightwall section of the optical fiber holder within the boot 38.This is dependent upon the choice of materials used for the boot 38 andthe optical fiber holder 18, and yields a design that does not require acrimp ring and which assists in ease of manufacture and reduces thenumber of needed components. "One-way" barbs on the surface of theoptical fiber holder assist in the attachment of the boot 38 to theoptical fiber holder 18, which also assists in the attachment of thecable to the plug 12.

A latch 40 is integrally molded onto one side of shroud 24 to releasablysecure plug 12 to receptacle 14. The latch 40 may also impart mechanicalpolarization to the plug, that is, it may be inserted into receptacle 14only in one orientation. Plug 12 may be biased in the interconnectedposition, e.g., by a springboard (a flexible cantilever) formed insidereceptacle 14, to be pushed back against latch 40 to minimize the effectof manufacturing tolerances.

The receptacle 14 includes a body or housing 70 and another fiber holder72. The housing 70 may also have appropriate features (such as latcharms 76) allowing it to be releasably mounted to bulkhead 16 which maybe, for example, a patch panel or workstation outlet (wall boxfaceplate). The latch mechanism may provide for mounting from the frontof the panel, to allow all preparatory work to be done at the front sideof the panel, or may provide for mounting from the rear of the panel, toallow all preparatory work to be done at the back side of the panel.Additional mechanisms may be provided, such as the fiber hold-down, toretain the fibers firmly in the grooves. The fibers do not extend to thevery tips of fingers 82 and 84 but rather terminate a sufficientdistance from the tips to allow proper support of the portion of theoptical fibers in the plug when the connector is in use. If thefiber-to-fiber contact occurs very near the tips of the V-grooves (or ifthe plug is inserted too far), the fiber portion in the plug can bendbeyond the groove and lifted away from the apex, breaking theconnection.

The receptacle 14 may have as many of these fingers with fiber-alignmentgrooves as there are fibers in plug 12. Fingers 82 and 84 are shaped toproject into slots 54 and 56, respectively, of shroud 24 when plug 12 isfully inserted into receptacle 14. Fingers 82 and 84 enter shroud 24 atan oblique (nonzero) angle with respect the plug axis, i.e., the axisdefined by either of the optical fibers 30 or 32 when they are extendingstraight within shroud 24. This angle is preferably about 42°, whichbalances concerns regarding fiber end face contact pressure, fiberforces directed into the V-groove, the effects of friction, and thedesired tolerance window (a larger angle increases tolerances). Sincethe receptacle fibers are not directed toward opening 74, there is nodanger of escaping light injuring a user's eyes. Receptacle fiber holder72 is pivotally attached to housing 70 by providing posts on the firstend of holder 72 which snap into cutouts or hooks 88 formed at one endof receptacle housing 70. The holder 72 releasably locks into placeusing bumps or studs formed on the side of the holder, which engageholes 89 in receptacle housing 70. An alternative design for thereceptacle fiber holder may be used in which the holder is molded as asingle piece with a breakaway top or cover plate that can snap onto itsbase, the base having the fiber-positioning grooves.

All of the components of connector 10 (except plug boot 38) may beformed of any durable light transmitting material, preferably aninjection moldable polymer such as polycarbonate, VALOX (a polyestersold by General Electric), or RADEL (a polyarylsulfone sold by Amoco).The material may include conductive fillers to render the componentssemiconductive in order to minimize triboelectric charging which caninduce fiber end contamination, so long as such fillers do not undulyattenuate the radiation during curing. The boot 38 is preferably formedof low modulus copolyester elastomer, such as that available from RTP ofWinona, Minn., under material number 1559X67420B.

Assembly and installation of connector 10 is straightforward. Plug 12 istypically assembled in the factory, although it may easily be assembledin the field. To place the adhesive into the assembly, simply placeadhesive into a syringe like applicator (if desirable, the adhesive maybe provided in such an applicator), over the opening in the holder andinject the adhesive into the holder 18. The adhesive is then cured byplacing a light having the required wavelength radiation above theconnector for a period of 25 seconds to a few minutes. Useful lightsinclude the Model XL3000, available from 3M, which uses a 75 Watttungsten source, either unfiltered or filtered to pass blue light.

It is also understood that plug 12 or receptacle 14 could be mounted ona jumper cable or patch cord with any kind of optical connector at theother end of the fibers. It is recommended that fibers be used whichhave a longer life when exposed to indoor environments, such as thehigh-strength fibers available from Minnesota Mining and ManufacturingCo. Those fibers have a conventional core and cladding which issurrounded by a novel three-layer construction, as discussed in U.S.Pat. No. 5,381,504, disclosed herein by reference. Those skilled in theart will also appreciate that the connector of the present invention canaccommodate discrete optical fibers or multifiber ribbons, as well asboth singlemode and multimode fibers.

Fibers which are to be pre-terminated to either plug 12 or receptacle 14should be stripped, cleaved and cleaned. If the fibers are in the formof a ribbon which is part of a bundled group of ribbons in a cable, thena portion of the cable jacket must first be cut back to reveal theribbons. Most cables have several protective layers, and each of theselayers must be removed to provide access to the fiber ribbons. Similarsteps must be taken to remove the protective layers of a cable having asingle discrete fiber. After the fibers have been removed from theprotective cable jacket, they are stripped. The stripped fibers are thenready for cleaving which may be accomplished using any one of severalcommercially available fiber cleavers, such as that shown in U.S. Pat.No. 5,024,363. The cleave length for attachment of the fibers to plug 12is the distance from fiber holder 18 which, in the preferred embodiment,is about 23 mm. For attachment of fibers to receptacle 14, the cleavelength is the distance from fiber holder 72 which, in the preferredembodiment, is about 15 mm.

Once the craftsperson is satisfied that each of the fibers has anacceptable end face, the fibers may be removed from the cleaver. Thefibers may further optionally be provided with an asymmetric treatment,like cleaving so as to impart an angled end face, as taught in U.S. Pat.No. 5,048,908. For the plug, fiber preparation may be done after thefiber cable has been threaded through boot 38.

Final assembly of plug 12 comprises the simple steps of placing thefibers in the V-grooves of holder 18 and snapping shroud 24 onto holder18. An assembly fixture may be used to guide shroud 24 onto the fiberholder so as to avoid damaging the fibers as they are inserted into theshroud. The ends of the fibers should terminate in the plug about 0.5 mmfrom the end of the shroud. Completion of receptacle 14 is also simple.Fiber holder 72 is attached to housing 70, first by pushing the pivotposts into cutouts 88, and then snapping the studs into holes 89. Careshould be taken during placement of the fibers in the V-grooves andattachment of the holder to the receptacle to not contaminate the fibertips.

Installation of connector 10 is equally straightforward. Receptacle 14is optionally mounted to any desired surface by convenient means, suchas latching arms 76 (other constructions could be molded into housing 70for custom mounting). Several receptacles could also be mounted in asingle module, and they can be designed for front or rear loading, orsliding from the side. After receptacle 14 is mounted, the connection iscompleted by simply inserting plug 12 into opening 74. Plug 12 isreleased from receptacle 14 by latch 40.

The dimensions of the various components of connector 10 may varyconsiderably depending upon the desired application. The followingapproximate dimensions are considered exemplary. Plug 12 has an overalllength of 57 mm, a width of 12 mm, and a thickness of 8 mm, and fiberholder 18 provides clamping grooves that are 13 mm long. Plug shroud 24extends 25 mm beyond holder 18, providing an interior space which is 24mm long, 10 mm wide and 6 mm high. Opening 74 of receptacle 14 is 12mm×10 mm. Its overall height and depth are 38 mm and 36 mm. Receptaclefiber holder 72 is 20 mm long (from the end where the fibers are clampedto the tips of fingers 82 and 84), 12 mm wide and 1.5 mm thick. Thefiber-alignment grooves in fingers 82 and 84 are 11.5 mm long and have amaximum depth of 2 mm which suitably accommodates most conventionaloptical fibers. The interior angle of the V-grooves should not be toonarrow since this might result in excess friction with the fibers, butit also should not be too wide since this would not keep the fibersguided properly. A 90° interior angle is believed to be a goodcompromise.

Although the invention has been described with reference to specificembodiments, this description is not meant to be construed in a limitingsense. Various modifications of the disclosed embodiment, as well asalternative embodiments of the invention, will become apparent topersons skilled in the art upon reference to the description of theinvention. Variations are possible in the plug, receptacle and holder.For detailed discussion of the mechanical construction of a usefulconnector, see U.S. Pat. No. 5,381,498, and copending U.S. Ser. Nos.08/577,740, and 08/801,058, all of which are incorporated herein byreference. For example, although only two fiber pairs are shownconnected in the figures, connector 10 could accommodate practically anynumber of fibers (or just a single pair). It is therefore contemplatedthat such modifications can be made without departing from the spirit orscope of the present invention as defined in the appended claims.

Test Methods Kevlar® Pull Test

Cables were inserted into the plug with no Kevlar® fanout. The jacketwas placed about 0.010 cm behind the rear injection ports. The adhesivewas injected by hand using a 3 cc syringe, being sure that the cavitywas completely filled.

Each plug was then cured with two 5 second exposures, one on the top andone on the bottom with the appropriate light sources (Dymax adhesive UV,or visible with blue filter, adhesive of the invention with whitelight).

Each cable was cut in half and split until two independent cablesprotruded from the plug. For the Kevlar® pulls, the plug was placed intoa fixture that was attached to a chattillon DFM100 scale. One at a time,the cables consisting of jacket, Kevlar®, and GGP fiber, were wrappedaround a 6.25 cm diameter mandrel and clamped into place. They werepulled at a rate of 1.25 cm per minute until failure.

Fiber Pull Test

For three fiber pulls, after the plug was placed into the fixture asdescribed for the Kevlar® pull test, the jacket was striped off aboutfour inches from the plug, exposing the Kevlar® and GGP fiber. The GGPfiber was wrapped around the 6.25 cm mandrel and clamped in place. Theywere pulled at a rate of 1.25 cm per minute until failure.

Next, the Kevlar® was wrapped around the 6.25 cm mandrel and pulled atthe same rate until failure.

All pull test were accurate to 0.1 pound, or 2.2 kilograms.

Glossary

    ______________________________________                                        CPQ       Camphorquinone                                                      EDMAB     Ethyl-p-dimethylaminobenzoat                                        DPIPF     diphenyliodonium hexafluorophosphate                                Bis GMA   Reaction product of Bis Phenol A diglycidyl ether                             and 2 equivalents of methacrylic acid                               TEGDMA    Triethyleneglycoldimethacrylate                                     DHEPT     diphenyliodonium hexafluorophosphate                                ______________________________________                                    

EXAMPLES Example 1

Samples of optical fiber assemblies were made by stripping the outerjacket of the cable, trimming the Kevlar® fibers and inserting theoptical fiber into a dual termination connector.

Before bonding the fibers were cleaned with a lint free pad which hadbeen wetted with isopropyl alcohol, then the fibers were inserted intothe fitting. Two drops of adhesive were then added to each fiber'schannel in the dual termination fixture, and the connector was bonded byexposing it for 25 seconds to a 3M Model XL 3000 curing light having a75 watt tungsten source filtered to pass blue light.

In Examples C1 and C2, the adhesives used were commercially available UVcurable adhesives from Dymax Corp, Torrington Conn., as Dymax 3072 andDymax 3095, respectively. The recommended curing light, Model 3000-ECwas also purchased from Dymax Corp. The adhesive of the invention hasthe following composition (Example 1).

    ______________________________________                                        INGREDIENT        PERCENT                                                     ______________________________________                                        HEMA w/ 10 ppm MEHQ                                                                             37.08%                                                      BISGMA resin      61.79%                                                      EDMAB              0.50%                                                      Camphoroquinone    0.25%                                                      DHEPT              0.38%                                                      ______________________________________                                    

The pull force needed to remove the fiber from the connector is shown inTable

                  TABLE 1                                                         ______________________________________                                               Kevlar ® Pull force                                                                      Fiber Pull force                                                                         UV cure time                                 Example                                                                              (Newtons.)     (Newtons.) (sec.)                                       ______________________________________                                        C1      67            6.6        40                                           C2     289            6.6        30                                           1      334            13         15                                           ______________________________________                                    

Examples 2-X and Comparative Examples

Samples were made according to Example 1 and tested for strength of theunified structure. In Examples 2 and 3, an adhesive of the inventionhaving a formulation as described below was used. These connectors werethen bonded by exposing them to 25 seconds to a 3M Model XL 3000 curinglight having a 75 watt tungsten source.

For Comparative Examples C4 and C5, commercially available adhesiveswere used; C6 was used as purchased, C4 was modified by the addition ofthe ternary photoinitiator system used in Examples 2 and 3, i.e., theCPQ, EDMAB, and DDPIPF6 in the same amounts. Example C5 included thismodification, and further containing 0.1% rose bengal.

Example

    ______________________________________                                        INGREDIENT    PARTS BY WEIGHT                                                 ______________________________________                                        Bis GMA       50.0                                                            TEGDMA        50.0                                                            CPQ           0.17                                                            EDMAB         1.00                                                            DPIPF6        0.60                                                            Tinuvin ® P                                                                             1.00                                                            ______________________________________                                    

Example

    ______________________________________                                        INGREDIENT    PARTS BY WEIGHT                                                 ______________________________________                                        Bis GMA       50.0                                                            TEGDMA        50.0                                                            CPQ           0.17                                                            Rose Bengal   0.10                                                            EDMAB         1.00                                                            DPIPF6        0.60                                                            Tinuvin ® P                                                                             1.00                                                            ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                               UV           Blue        White                                                Kevlar ®                                                                           Fiber   Kevlar ®                                                                         Fiber                                                                              Kevlar ®                                                                         Fiber                                     Pull     Pull    Pull   Pull Pull   Pull                                      force    force   force  force                                                                              force  force                              Example                                                                              (N.)     (N.)    (N.)   (N.) (N.)   (N.)                               ______________________________________                                        C4     207.1    3.41    302.3  3.85 298.4  2.96                               C5     252.8    3.85    311.5  3.41 15.96  3.11                               C6     48.45    3.26    8.668  3.41 15.96  3.11                               2      320.3    7.26    342.6  8    343.7  N/A                                3      345.9    5.93    334.3  8.59 351.5  9.63                               ______________________________________                                    

As can be seen, the commercially available adhesive does not produce thesame type of unified structure as adhesives of the invention, even whena ternary photoinitiator and additional sensitizers are added. This istrue whether ultraviolet light is used to cure the adhesives, visiblelight across a broad spectrum, or visible light filtered to pass onlythe blue area of the spectrum.

What is claimed is:
 1. A method for forming a unified structure in a fiber optic connector comprising at least one fiber optic cable, and a holder therefor, each cable containing at least one optical fiber therein, surrounded by a plurality of strengthening fibers, both said optical fiber and said strengthening fibers being surrounded by at least one polymeric coating layer, said method comprising the steps ofa) stripping a portion of said cable in a layerwise manner such that a terminal end of exposed fiber is succeeded by an area of exposed strengthening fibers, b) placing said cable into said holder, c) injecting a visible light curable adhesive into said holder, d) exposing said adhesive to visible light having a wavelength of between 400 nm and 600 nm for a period of time from about 5 seconds to about 30 seconds.
 2. A method according to claim 1 wherein said adhesive is curable by radiation having a wavelength between about 500 and about 600 nanometers.
 3. A method according to claim 1 wherein said adhesive comprises at least one acrylate monomer.
 4. A method according to claim 3 wherein said adhesive contains at least one monomer selected from the group consisting of hydroxypropyl methacrylate, hydroxyethyl methacrylate (HEMA), and methacrylic acid.
 5. A method according to claim 1 wherein said adhesive comprises a photoinitiator system containing an electron donor and at least one sensitizer.
 6. A method according to claim 5 wherein said photoinitiator system comprises a fluorenone sensitizer.
 7. A method according to claim 5 wherein said photoinitiator system further comprises a diaryliodonium salt.
 8. A method according to claim 1 wherein said comprises a plurality of fiber optic cables.
 9. A method according to claim 1 wherein said fiber optic cable is a multifiber ribbon.
 10. A fiber optic connector including a unified structure, said structure comprisinga) at least one fiber optic cable comprising at least one optical fiber surrounded by a plurality of strengthening fibers, both of which are surrounded by at least one polymeric coating layer, a portion of said cable being stripped in a layerwise manner such that a terminal end of exposed fiber is succeeded by an area of exposed strengthening fibers, succeeded by an area of coated cable; b) a holder for said optical cable; and c) a visible light curable adhesive injected into said holder, said adhesive comprising a photoinitiator system containing an electron donor and at least one sensitizer; wherein said optic fiber, said strengthening fibers and said holder are bonded into said unified structure by means of exposure to visible light for a period of up to about 30 seconds.
 11. The fiber optic connector of claim 10, wherein said adhesive is curable by radiation having a wavelength between about 400 and about 600 nanometers.
 12. The fiber optic connector of claim 10, wherein said adhesive is curable by radiation having a wavelength between about 500 and about 600 nanometers.
 13. The fiber optic connector of claim 10, wherein said adhesive comprises at least one acrylate monomer.
 14. The fiber optic connector of claim 13, wherein said adhesive contains at least one monomer selected from the group consisting of hydroxypropyl methacrylate, hydroxyethyl methacrylate, and methacrylic acid.
 15. The fiber optic connector of claim 10, wherein said photoinitiator system comprises at least one fluorenone sensitizer.
 16. The fiber optic connector of claim 10, wherein said photoinitiator system further comprises a diaryliodonium salt.
 17. The fiber optic connector of claim 10, wherein said adhesive cures in from about 5 seconds to about 30 seconds.
 18. The fiber optic connector of claim 10, comprising a plurality of fiber optic cables.
 19. The fiber optic connector of claim 10, wherein said optical cable is a multifiber ribbon.
 20. A fiber optic connector including a unified structure, said structure comprisinga) at least one fiber optic cable comprising at least one optical fiber surrounded by a plurality of strengthening fibers, both of which are surrounded by at least one polymeric coating layer, a portion of said cable being stripped in a layerwise manner such that a terminal end of exposed fiber is succeeded by an area of exposed strengthening fibers, succeeded by an area of coated cable; b) a holder for said optical cable; and c) a visible light curable adhesive injected into said holder, said adhesive comprising at least one acrylate monomer; wherein said optical fiber, said strengthening fibers and said holder are bonded into said unified structure by means of exposure to visible light for a period of up to about 30 seconds.
 21. The fiber optic connector of claim 20, wherein said adhesive is curable by radiation having, a wavelength between about 400 and about 600 nanometers.
 22. The fiber optic connector of claim 20, wherein said adhesive is curable by radiation having a wavelength between about 500 and about 600 nanometers.
 23. The fiber optic connector of claim 20, wherein said adhesive contains at least one monomer selected from the group consisting of hydroxypropyl methacrylate, hydroxyethyl methacrylate, and methacrylic acid.
 24. The fiber optic connector of claim 20, wherein said photoinitiator system comprises at least one fluorenone sensitizer.
 25. The fiber optic connector of claim 20, wherein said photoinitiator system further comprises a diaryliodonium salt.
 26. The fiber optic connector of claim 20, wherein said adhesive cures in from about 5 seconds to about 30 seconds.
 27. The fiber optic connector of claim 20, comprising a plurality of fiber optic cables.
 28. The fiber optic connector of claim 20, wherein said optical cable is a multifiber ribbon. 